Ceramic tubes have found a use in the manufacture of Solid Oxide Fuel Cells (SOFCs). There are several types of fuel cells, each offering a different mechanism of converting fuel and air to produce electricity without combustion. In SOFCs, the barrier layer (the “electrolyte”) between the fuel and the air is a ceramic layer, which allows oxygen atoms to migrate through the layer to complete a chemical reaction. Because ceramic is a poor conductor of oxygen atoms at room temperature, the fuel cell is operated at 700° C. to 1000° C., and the ceramic layer is made as thin as possible.
Early tubular SOFCs were produced by the Westinghouse Corporation using long, fairly large diameter, extruded tubes of zirconia ceramic. Typical tube lengths were several feet long, with tube diameters ranging from ¼ inch to ½ inch. A complete structure for a fuel cell typically contained roughly ten tubes. Over time, researchers and industry groups settled on a formula for the zirconia ceramic which contains 8 mol% Y2O3. This material is made by, among others, Tosoh of Japan as product TZ-8Y.
Another method of making SOFCs makes use of flat plates of zirconia, stacked together with other anodes and cathodes, to achieve the fuel cell structure. Compared to the tall, narrow devices envisioned by Westinghouse, these flat plate structures can be cube shaped, 6 to 8 inches on an edge, with a clamping mechanism to hold the entire stack together.
A still newer method envisions using larger quantities of small diameter tubes having very thin walls. The use of thin walled ceramic is important in SOFCs because the transfer rate of oxygen ions is limited by distance and temperature. If a thinner layer of zirconia is used, the final device can be operated at a lower temperature while maintaining the same efficiency. Literature describes the need to make ceramic tubes at 150 μm or less wall thickness.
An SOFC tube is useful as a gas container only. To work it must be used inside a larger air container. This is bulky. A key challenge of using tubes is that you must apply both heat and air to the outside of the tube; air to provide the O2 for the reaction, and heat to accelerate the reaction. Usually, the heat would be applied by burning fuel, so instead of applying air with 20% O2 (typical), the air is actually partially reduced (partially burned to provide the heat) and this lowers the driving potential of the cell.
An SOFC tube is also limited in its scalability. To achieve greater kV output, more tubes must be added. Each tube is a single electrolyte layer, such that increases are bulky. The solid electrolyte tube technology is further limited in terms of achievable electrolyte thinness. A thinner electrolyte is more efficient. Electrolyte thickness of 2 μm or even 1 μm would be optimal for high power, but is very difficult to achieve in solid electrolyte tubes. It is noted that a single fuel cell area produces about 0.5 to 1 volt (this is inherent due to the driving force of the chemical reaction, in the same way that a battery gives off 1.2 volts), but the current, and therefore the power, depend on several factors. Higher current will result from factors that make more oxygen ions migrate across the electrolyte in a given time. These factors are higher temperature, thinner electrolyte, and larger area.
Fuel utilization is a component of the overall efficiency of the fuel cell. Fuel utilization is a term that can describe the percent of fuel that is converted into electricity. For example, a fuel cell may only convert 50% of its fuel into electricity, with the other 50% exiting the cell un-used. Ideally, the fuel utilization of a fuel cell would be 100%, so that no fuel is wasted. Practically, however, total efficiency would be less than 100%, even if fuel utilization was 100%, because of various other inefficiencies and system losses. Additionally, if the gas molecules can't get into and out of the anode and cathode, then the fuel cell will not achieve its maximum power. A lack of fuel or oxygen at the anodes or cathodes essentially means that the fuel cell is starved for chemical energy. If the anode and/or cathode are starved for chemicals, less power will be generated per unit area (cm2). This lower power per unit area gives lower total system power.
In a tubular fuel cell device, such as that shown in FIG. 1 where the anode lines the inside of the tube and the cathode forms the outer surface with the electrolyte therebetween, it is wishful thinking to expect high utilization of fuel. The inside diameter of the tube, which forms the fuel passage, is very large when compared to the thickness of the anode. Anode thicknesses may be on the order of 50-500 nm, whereas tube diameters may be on the order of 4-20 mm. Thus, there is a high likelihood of fuel molecules passing through the large fuel passage without ever entering the pores of the anode. An alternate geometry for the tube is to have the anode on the outside of the tube. In that case, the problem could be worse because the fuel is contained within the furnace volume, which is even larger than the volume within the tube.
Within a multilayer fuel cell device, such as the Fuel Cell Stick™ devices 10 depicted in FIGS. 2 and 3 and developed by the present inventors, fuel utilization can be higher because the flow path for the gas can be smaller. FIG. 2 is identical to FIG. 1 of U.S. Pat. No. 7,838,137, the description of which is incorporated by reference herein. Device 10 includes a fuel inlet 12 feeding a fuel passage 14 to a fuel outlet 16, and an oxidizer inlet 18 feeding an oxidizer passage 20 to an oxidizer outlet 22. An anode 24 is adjacent the fuel passage 14 and a cathode 26 is adjacent the oxidizer passage 20, with an electrolyte 28 therebetween. By way of example, both the anodes 24 and fuel passages 14 can be made to a thickness of 50 nm, and this similarity in thickness, where the ratio of thickness can be near 1:1 (or a bit higher or lower, such as 2:1 or 1:2) can give a more optimal chance of molecule flow into and out of pores.
These multilayer fuel devices 10 are built from green materials, layer by layer, and then laminated and co-fired (sintered) to form a single monolithic device having a ceramic support structure 29 surrounding one or more active cells 50, each active cell 50 having an associated anode 24, cathode 26 and electrolyte 28 fed by fuel and air passages 14, 20. An active cell 50 (or active layer 50) is one in which an anode 24 is in opposing relation to a cathode 26 with an electrolyte 28 therebetween, and the active passages are those that run along or within the active cell 50. FIG. 3 depicts two active cells 50. Areas of the device 10 that lack an opposed anode 24 and cathode 26 are non-active or passive portions of the device 10 that form the support structure 29, and passive gas passages are those that run through these passive portions of the device 10. The active cells 50 are “within” the device 10 and substantially surrounded by and supported by ceramic support structure 29. The device has an exterior surface and internal supporting structure, which is the ceramic support structure 29, such that the active cells 50 are contained substantially inward of the exterior surface and are contained by the internal ceramic support structure. It should be understood that extension of all or a portion of an electrode to an edge of the device for electrical connection at the exterior surface does not compromise the support of this structure as the active cell 50 is still within the interior structure, and is within the scope of “substantially surrounded.” The electrolyte 28 in the active cell 50 is monolithic with the ceramic support structure 29 by virtue of being co-fired therewith, and may be made of the same or different material. In exemplary embodiments, the electrolyte 28 and ceramic support structure 29 are the same or similar in composition, with the primary difference between them being that the electrolyte 28 is that portion of the ceramic material that lies between an opposing anode 24 and cathode 26 (i.e., the middle layer in the 3-layer active cell 50) and the ceramic support structure 29 is the remaining portion of the ceramic material (i.e., the ceramic that surrounds the 3-layer active cell 50). Air and fuel are fed into the device 10 through the passive passages that are fluidicly coupled to the active passages that feed the active cells 50. Thus, a fuel passage 14 and an oxidizer passage 20, as referred to herein, include both the passive and active portions of the passages.
As discussed above, it is desirable to make the electrolyte 28 as thin as possible. However, as the electrolyte 28 is made thinner, the support of the structure can be compromised, and distortion of the active portion of fuel and air passages 14, 20 that feed the anodes 24 and cathodes 26 can occur at one or more locations within the active cell 50, as well as distortion of the passive portions of the passages 14, 20. These distortions in the passages 14, 20 may lead to leaks that degrade the performance of the affected active cell 50 and of the overall device 10.
One advantage of the multilayer fuel cell devices developed by the present inventors is that many active cells 50 can be provided within a single monolithic device, including multiple cells along a single active layer and stacks of active layers one upon another, which can be connected in various parallel and series arrangements, leading to a single device with high output. If one area of one cell distorts, there are still many other cells that produce power, such that the multilayer fuel cell devices are still superior to single cell tubular devices or stacked devices that are not monolithic. However, the more layers that are incorporated, the higher the chance for multiple distortions throughout the device.
Therefore, there is a need to provide thin electrolyte layers while still providing the needed support to prevent distortion of the gas passages within a monolithic multilayer fuel cell device.